Recent studies have confirmed the airborne occurrence of ragweed pollen allergens in natural aerosols substantially smaller than intact grains. Since these agents may add substantially to overall exposure burdens, preferentially challenge the subglottic airways and display other distinctive properties, we have proposed to study them intensively. Small micronic and submicronic aerosols will be collected by differential filtration using molecular membranes and by condensation of atmospheric moisture. Deposits will be assayed for major pollen allergens using lgE- and lgG-based ELISA procedures. Special interest will be focused on allergens of local ragweeds, grasses, oaks and fungi (e.g. Alternaria, Cladosporium). In addition, evidence of long-distance transport of microaerosol allergens will be sought and prevalence patterns correlated with diurnal climatic and seasonal variables. Potential disperson of allergenic macro- and microaerosols from short ragweeds will be studied using extraseasonally reared plants and possible direct scouring of allergen by air from pollens tested. In addition, we will evaluate the possibility, by direct challenge, that submicronic aerosols or osmyls may be the basis of some adverse respiratory reactions to natural Christmas trees, old book collections and mouldy interior surfaces.